A flow meter employing a specialized sensor for capturing changes in the flow speed of a gas along a flow path, such as a hose through which the gas is flowing, is used to measure of respiratory volume, a value which is employed in the fields of medical treatment and physiological research. Respiratory flow meters such as these are (1) directly applied to the mouth of a person, (2) incorporated into the inhalation or exhalation duct system, etc., and are used for obtaining measurements in the case where the subject is a human being confined in a room where movement and activities are minimal. This type of flow meter device is not appropriate for measurements in the case where the subject is a human being who is exercising or performing activities that are accompanied by movement. In addition, in order to measure the respiratory volume of a user who is wearing the breathing apparatus, the gas circuit such as the arrangement of the piping and devices for measuring results in a large device. As a result, the device cannot be made portable for the user. Furthermore, it has been technically difficult to employ the aforementioned flow meters to measure respiratory volume in breathing apparatuses provided with a demand pressure regulator, in which respiratory gas stored at high pressure is inhaled during breathing.
A method has been attempted in which lung capacity, which is a primary factor in determining respiration in humans and animals, is estimated based on changes in form as a method for measuring respiratory volume without employing a flow meter. However, from the perspective of accuracy and practical application in the water or under other such specialized conditions, this method has not yet reached the point where it can be used in the field.
On the other hand, dive computers have been developed in recent years for scuba diving with the intention of making diving safer by preventing decompression sickness. Among these devices, there are those that measure the gas pressure (residual pressure) in the high pressure gas container. However, these devices have as their main objective the display of the gas remaining and the provision of a warning to the user, and lack the fine sensitivity or accuracy for measuring gas consumption per breath taken by the diver.
In any case, the conventional technology has not yet provided a device for directly measuring the volume of the gas itself as an indicator of the respiratory gas consumption value.